vayne has a hard early game, which in solo queue results in low lane presence which means you have little influence in the early game to create a win. you are basically rolling the dice to see if you get to end game, in which if you do the odds of you winning greatly increase

Please don't pick Vayne in ranked, it just doesn't work, honestly unless you are Doublelift who just plays an insane Vayne and you have a team that can build around it as well as not just give up when you fall behind by a kill or two it won't work. Also it is impossible to do anything with her when your Support is an idiot. Hyper Carries like Kog' Maw or Twitch, or a lesser hyper carry like Tristana are all way better on the Yolo Q. With Vayne the game usually just ends before you can do anything and the team gives up when you are down only like 4 kills even though you have a hyper carry, it is dumb.